That behaviour would indicate that the battery actually is fully charged already at that point, when the LED changes from red to blue (but not flashing back and forth!) with the AC adapter connected the projector will be powered directly from the AC adapter, bypassing the battery.
Is the battery percentage reported close to 100% when the LED turns blue? If it’s not I believe the percentage is wrong and the reporting may become more accurate if you discharge fully as described in Charging/battery issue - #19 by Philips_Support_P